The present invention relates to a pneumatically operated screw driver providing an axially driving force by a piston and rotational force by a pneumatic motor for screwing a threaded fastener into a woody member or the like.
U.S. Pat. No. 6,026,713 discloses a pneumatically operated screw driver including a driver bit engageable with a groove formed in a head of the faster. The driver bit is connected to a piston which is driven in an axial direction of the drive bit upon application of a pneumatic pressure to one side of the piston. Further, a pneumatic motor is provided for rotating the piston about its axis. Thus, the driver bit is axially movable while being rotated about its axis for screwing the fastener into a target. Further, a bumper is provided so as to absorb kinetic energy of the piston moving to its bottom dead center. An operation valve associated with a trigger is provided for opening a main valve in order to apply pneumatic pressure onto the piston.
The disclosed screw driver also includes a return chamber to which a compressed air is accumulatable for applying compressed air to the piston in order to move the piston and the driver bit to their initial positions. More specifically, accumulation of the compressed air into the return chamber is started when the piston is about to reach its bottom dead center. When the screw fastening operation is terminated upon abutment of the piston onto the bumper, the compressed air accumulated in the return chamber will be applied to an opposite side of the piston so as to return the piston and the driver bit to their original positions.